The present invention relates to an improved electrostatic copying apparatus. Such an apparatus comprises a photoconductive belt which is electrostatically charged and radiated with a light image while held stationary. This results in the formation of an electrostatic image which is developed with a toner substance to form a toner image which is transferred and fixed to a copy sheet.
In order to produce copies which are identical to the original documents, it is necessary to optimally adjust various operating parameters such as developing bias voltage and the like in accordance with the characteristics of the document. Generally, the developing bias voltage will vary in accordance with the optical density and contrast of the document.
In the type of copying apparatus to which the present invention relates, prior art attempts to optimally adjust the developing bias voltage include prescanning or sensing the entire document to determine the optical density pattern and adjusting the bias voltage in accordance with the overall density and contrast of the pattern. However, this involves the drawback that the prescanning requires an additional amount of time and therefore significantly reduces the copying speed. Also, the life of the illumination lamp is decreased and heat generation and power consumption are increased.
Another expedient is to sense the optical density of the leading edge portion of the document during scanning and adjust the bias voltage in accordance therewith. However, the leading edge portion of the document may vary considerably from the remainder of the document and the bias voltage may be adjusted erroneously.
Other problems inherent in prior art copying apparatus include unnecessary deterioration of the belt and overloading of a cleaning unit therefor due to toner adhering to non-image areas of the belt and the necessity of manually selecting a copy sheet size in accordance with the size of the original document.